[Techtalk] formatting half a mirrored raid...

Conor Daly conor.daly at oceanfree.net
Fri Jun 13 21:31:48 EST 2003

On Fri, Jun 13, 2003 at 11:57:56AM -0400 or so it is rumoured hereabouts, 
Walt thought:
> Also, I would expect the system to start
> working considerably to copy over the 110 gb
> of data, but it's doing nothing. And yes, I'm
> using the exact same raidtab.

I don't think it actually needs to copy over any data.  You did a mke2fs
on the disk but didn't actually overwrite any data.  AIUI, the raid resync
process works at a datablock level (or something) and, since non of the
_datablocks_ were deleted, it doesn't need to write any data.  Instead, it
just checks the validity of it in some other fashion.  Also, the raid
resync self limits to some max transfer rate and further throttles that
depending on system load.  I've seen one throttle from a rate of
100Mb/unit time to 100b/unit time so the users don't see much drag.
> Conor Daly wrote:
> >You should consider your raid types you are using here.  Your md1 is raid0
> >which gives you no redundancy.
> Yes, I am pondering this decision... Each 73 gb
> drive costs c. $450... So adding a parity drive is
> fairly expensive, and I need all the space I can
> get. We eating up about 10-15 gb a month +/-.
> Plus, my plan is to eventually use the 120gb
> drives as backup for the striped scsi raid.
> But this brings up another question: can I add a
> parity drive and switch to another raid level at a
> later day? Or now that I have it setup as raid0,
> I'd have to start over with blank drives to switch
> to level 3 and copy the data all over again?

Yes.  raid3 is essentially raid0 + parity disk.  You should be able to add

parity_disk	/dev/sdd1

to your raidtab, change the raidlevel to '3' and raidstop, raidstart the

OTOH, if you want raid5 which stripes the parity info across all disks[0],
you'll have to wipe and copy.

> >If, OTOH, you use "linear" for this raid set, the disks will
> >simply be concatenated and loss of a disk will mean loss of _only_ the
> >data stored on that disk.
> Linear? "And he learned *new* things to say!"
> How do I make it linear? Arg! Is that another
> raid level or just a setting for level 0 raid?

Another level.  It's detailed in the Software-RAID HOWTO section 4.
Basically, its main function is to allow different sized disks to be
concatenated[1].  Contrary to my earlier claim, loss of a disk in a linear
array means loss of the _entire_ array so it's a poorer option than raid0


[0] In raid3, the write speed of the parity disk becomes a bottleneck
    since _every_ disk write uses it.  Works well where the array is
    primarily read-only.  In raid5, writes are striped to all disks and
    therefore performs better where many disk writes occur.

[1] Handy for using those old 2GB disks...

[2] raid0 parallels disk reads over many disks so throughput is good.
    Linear reads happen from one disk only at a time.
Conor Daly <conor.daly at oceanfree.net>

Domestic Sysadmin :-)
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